In my U.S. Pat. No. 4,136,602 issued on Jan. 30, 1979 and entitled "Hydraulic Motor" a low speed high torque motor was described which included a drive assembly having a plurality of fluid driven pistons and a pair of cam riders mounted on each piston and positioned to engage a cam surface having a predetermined or fixed cam configuration. The fixed cam surface determined the speed and torque of the motor depending upon fluid pressures. Variations in speed-torque being achieved by varying the fluid pressure. If a different speed-torque characteristic was required at a predetermined fluid pressure, the fixed cams had to be changed.